VIPER25HDTR datasheet(16/40 Pages) STMICROELECTRONICS

16 / 40 page

Operation description

VIPER25

16/40

Doc ID 15585 Rev 4

7.3

Power-up description

If the input voltage rises up till the device start level, VDRAIN_START, the VDD voltage begins to

grow due to the IDDch current (see Table 7 on page 7) coming from the internal high voltage

start-up circuit. If the VDD voltage reaches the VDDon threshold (See Table 7 on page 7) the

power MOSFET starts switching and the HV current generator is turned OFF, see Figure 23

on page 17.

The IC is powered by the energy stored in the capacitor on the VDD pin, CVDD, until when

the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode)

develops a voltage high enough to sustain the operation.

CVDD capacitor must be sized enough to avoid fast discharge and keep the needed voltage

value higher than VDDoff threshold. In fact, a too low capacitance value could terminate the

switching operation before the controller receives any energy from the auxiliary winding.

The following formula can be used for the VDD capacitor calculation:

Equation 1

The tSSaux is the time needed for the steady state of the auxiliary voltage. This time is

estimated by applicator according to the output stage configurations (transformer, output

capacitances, etc.).

During normal operation, the power MOSFET is switched ON immediately after transformer

demagnetization, detected by the VIPER25, through the voltage VZCD sensed on the ZCD

pin. At power up the initial output voltage is zero and then the voltage VZCD is not high

enough to correctly arm the internal ZCD circuit. In this case, the power MOSFET is turned

ON with a fixed frequency determined by the internal oscillator. This fixed switching

frequency is FSTARTER (see Table 8 on page 8). As soon as the voltage on ZCD pin is able

to arm the ZCD circuit (i.e. its positive value exceeds VZCDAth), the turn-on of the power

MOSFET is driven by this circuit and is no more related to the internal oscillator (except for

the frequency fold-back function).

The start-up phase is managed by a dedicated internal logic and is activated every time the

device exits from UVLO because the VDD voltage exceeds the threshold VDDon. An internal

timing (tSU, see Table 8 on page 8) defines the end of the start-up phase.

During the first part of the start-up phase soft start takes place: the drain peak current is

increased cycle-by-cycle from zero as far as the maximum value, IDlim, (see Figure 24 or

Figure 25 on page 18). The duration of soft-start is tSS, (tSS < tSU, see Table 8 on page 8),

During soft-start and until the output voltage reaches its regulated value, the feedback loop

is open. To prevent an improper activation of the OLP function (see the Section 7.13 on

page 28) during soft-start and until the start-up phase is over (t = tSU), the feedback voltage

is clamped at VFBlin.(see Figure 24 on page 18).

In this way, the feedback voltage can exceed VFBlin and ramp up as far as the overload

threshold, VFBolp (see Figure 25 on page 18), which would activate the OLP function, only at

the end of the start-up phase (t > tSU) if the output voltage is still below the regulated value.

DDoff

DDon

SSaux

DDch

VDD

−

⋅

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